1. Field of the Invention
This invention relates generally to the field of natural gas storage and handling and specifically to a natural gas vapor condensing system.
2. Description of the Related Art
As more stringent emissions standards are established for motor vehicles, the use of alternatives to gasoline and diesel fuels becomes increasingly desirable. Among the alternatives is natural gas, comprising primarily methane, which can be used in most internal combustion engines, often with minimal modification. Methane has also been used to some extent as a fuel for aircraft. Because the normal state of natural gas is gaseous, certain storage, delivery, and handling considerations arise. Compressed natural gas has been used with some success, but does not provide nearly the same energy density as gasoline. Liquified natural gas (LNG) provides greater energy density, but must be insulated and is ordinarily slightly pressurized. LNG held in storage tanks, located on ships or on land, is subject to evaporation due to heat leakage through insulation of the tanks. LNG handling and delivery systems for transferring LNG from one vessel to another are well known, but during transfer, some of the LNG evaporates and can be lost to the atmosphere. For example, during delivery from a bulk storage tank to a natural gas fueled vehicle tank or other small vessel, vapors are generated and may be lost and liquids may be spilled. Natural gas in the vehicle tank may also evaporate and be wasted. In any storage or transfer system, waste and spillage should be minimized due to efficiency and environmental factors.
Natural gas comprises primarily methane with various amounts of other components. The present description discusses the use of methane, but would apply to natural gas or any other substance including a significant amount of methane. "Fugitive" methane includes methane or natural gas that does not reach or is unusable at the intended destination, such as the vessels or tanks discussed above, and can be recovered or contained in a vaporous or liquid state. The fugitive methane may be vapors that are not efficiently delivered or result from spills or evaporation. It is desirable to recover the fugitive methane, which would otherwise be wasted. Fugitive vapors can be recovered by any of a number of known means. Once these vapors are recovered, it is desirable to return them to the supply of LNG for storage or use.
U.S. Pat. No. 4,846,862 to Cook, incorporated herein by reference, shows a system that uses a closed-loop nitrogen refrigeration cycle to reliquify LNG boil-off from a storage receptacle.
It is desirable to have a method and system that reliquifies or condenses natural gas or methane vapors by thermal contact with a sufficiently cold substance. In addition, the method should not introduce undesirable impurities into the natural gas.